U.S. patent number 5,541,627 [Application Number 07/809,191] was granted by the patent office on 1996-07-30 for method and apparatus for ejecting a droplet using an electric field.
This patent grant is currently assigned to Xerox Corporation. Invention is credited to Calvin F. Quate.
United States Patent |
5,541,627 |
Quate |
July 30, 1996 |
Method and apparatus for ejecting a droplet using an electric
field
Abstract
A method and apparatus for ejecting droplets from the crests of
capillary waves riding on the free surface of a liquid by
parametrically pumping the capillary waves with electric fields
from probes located near the crests. Crest stabilizers are
beneficially used to fix the spatial locations of the capillary
wave crests near the probes. The probes are beneficially switchably
connected to an AC voltage supply having an output that is
synchronized with the crest motion. When the AC voltage is applied
to the probes, the resulting electric field adds sufficient energy
to the system so that the surface tension of the liquid is overcome
and a droplet is ejected. The AC voltage is synchronized such that
the droplet is ejected about when the electric field is near is
minimum value. A plurality of droplet ejectors are arranged and the
AC voltage is switchably applied so that ejected droplets form a
predetermined image on a recording surface.
Inventors: |
Quate; Calvin F. (Stanford,
CA) |
Assignee: |
Xerox Corporation (Stamford,
CT)
|
Family
ID: |
25200748 |
Appl.
No.: |
07/809,191 |
Filed: |
December 17, 1991 |
Current U.S.
Class: |
347/10;
347/46 |
Current CPC
Class: |
B41J
2/06 (20130101); B41J 2002/061 (20130101) |
Current International
Class: |
B41J
2/04 (20060101); B41J 2/06 (20060101); B41J
029/38 (); B41J 002/135 () |
Field of
Search: |
;346/1.1,14R,75
;347/44,46,55,9,10,20 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
W Eisenmenger entitled "Dynamic Properties of the Surface Tension
of Water and Aqueous Solutions of Surface Active Agents with
Standing Capillary Waves in the Frequency Range From 10 kc/s to 1.5
Mc/s," pp. 327-340 of vol. 9, of Acustica, 1959..
|
Primary Examiner: Wong; Peter S.
Assistant Examiner: Frahm; Eric
Attorney, Agent or Firm: Kelly; John M.
Claims
What is claimed:
1. A method of ejecting a droplet from a liquid having a surface
tension and an attractive sensitivity to an electric field, the
method comprising the steps of:
generating a capillary wave having a crest and wave motion with a
predetermined frequency on a free surface of the liquid; and
parametrically pumping the liquid with an electric field such that
the surface tension is overcome and the droplet is ejected from
said crest.
2. The method according to claim 1, further including the step
of
stabilizing said crest in a predetermined position.
3. The method according to claim 2, wherein said step of
parametrically pumping the liquid includes the step of reducing
said electric field substantially to zero about when the droplet is
ejected.
4. A method of ejecting a droplet from a free surface of a marking
fluid having a surface tension and an attractive sensitivity to an
electric field onto a recording surface, the method comprising the
steps of:
generating a capillary wave having a crest and wave motion with a
predetermined frequency on the free surface of the marking fluid;
and
parametrically pumping the marking fluid using an electric field to
cause a droplet of the marking fluid in said crest to overcome the
surface tension and to be ejected onto the recording surface.
5. The method according to claim 4, further including the step
of
stabilizing said crest in a predetermined position.
6. The method according to claim 5, wherein said step of
parametrically pumping the marking fluid includes the step of
reducing said electric field substantially to zero about when the
droplet is ejected.
7. A droplet ejector for ejecting a droplet from a free surface of
a liquid of a type having a surface tension and an attractive
sensitivity to an electric field, comprising:
a container for containing the liquid;
means for generating a capillary wave having a crest and a wave
motion with a predetermined frequency on the liquid;
means for spatially stabilizing said crest in a position within
said container;
an electrically conductive probe disposed near said stabilized
crest position; and
supply means selectively coupled across said probe and said liquid
for selectively generating an attractive electric field from said
probe into said liquid which parametrically pumps said liquid to a
level sufficient to cause a droplet to overcome the surface tension
of the liquid and to be ejected from said crest.
8. The apparatus according to claim 7, wherein said supply means
creates a substantially zero electric field near the time the
droplet is ejected.
9. A printer of the type having a means for moving a print head
relative to a recording medium and a means for producing image
control signals associated with an image to be produced, wherein
the improvement is a print head comprising:
a container for containing a marking fluid having a surface tension
and an attractive sensitivity to an electric field;
means for generating a capillary wave having a crest and a
predetermined wave motion on the free surface of the marking
fluid;
means for stabilizing said crest in a predetermined position within
said container; and
probe means for generating a time varying electric field which
parametrically pumps the marking fluid such that a droplet is
ejected from said crest in response to the image control
signals.
10. The printer according to claim 9 wherein the marking fluid is
ink.
11. A print head for ejecting a marking fluid characterized by a
surface tension and by an attraction to an electric field,
comprising:
a container for containing the marking fluid such that said marking
fluid has a free surface;
means for generating a capillary wave having a crest on said free
surface of the marking fluid;
means for stabilizing said crest in a predetermined position within
said container;
probe means associated with said crest for producing an electric
field into the marking fluid local to said crest, said probe joined
to said container in a substantially fixed position and spatially
disposed adjacent the stabilized crest position; and
supply means selectively coupled across said probe means and said
marking fluid for selectively generating an attractive electric
field from said probe means into said marking fluid which
parametrically pumps said marking fluid to a level sufficient to
cause a droplet to overcome the surface tension of said marking
fluid and to be ejected from said crest.
Description
The present invention relates to droplet ejectors. More
particularly, it relates to methods and devices for ejecting
droplets from crests of capillary waves on the free surface of a
liquid by parametrically pumping the liquid with an electric field,
and the use of those methods and devices in drop-on-demand
printers.
BACKGROUND OF THE INVENTION
Many types of printers have been developed. The best printer to use
in a particular application depends on factors such as the
printer's relative cost, reliability, availability, speed,
recording medium, and marking techniques. However, when direct
marking on a recording medium is required, drop-on-demand printers
are an appropriate choice.
Numerous kinds of drop-on-demand printers are either available or
under development. For example, nozzle-based ink jet printers which
emit ink through a small nozzle or orifice have been available for
some time. Despite the work that has gone into developing these
printers, they remain subject to various problems such as nozzle
clogging; high production costs, which is at least partially a
result of the difficulty in producing the nozzles; and image
smearing, a result of using slowly drying ink to reduce clogging.
While various solutions to these and other problems have been
implemented or proposed, experience suggests that nozzle-based ink
jet printers are not optimum.
In view of the above, other types of drop-on demand printers have
been proposed. Far example, Kohashi in U.S. Pat. No. 4,383,265,
issued 10 May 1983, disclosed an electroosmotic ink recording
apparatus potentially usable for drop on demand printing. The '265
patent teaches the wicking of ink over an electrode using
electroosmosis and the subsequent inducing of ink to jump from the
wick onto a recording surface by using coulomb forces developed via
a second electrode behind the recording medium While the technology
found in the '265 patent may avoid some of the problems with
nozzle-based printers, its teachings have not achieved wide spread
use.
In any event, other drop-on-demand print technologies are being
developed. One such technology of special interest to the present
invention involves the use of capillary waves, specifically as
taught in U.S. Pat. Nos. 4,719,476 and 4,719,480, respectively
entitled "Spatially Addressing Capillary Wave Droplet Electors and
the Like," and "Spatial Stabilization of Standing Capillary Surface
Waves." Both patents issued to inventors Elrod, Khuri-Yakub, and
Quate on 12 Jan. 1988, and both are hereby incorporated by
reference.
U.S. Pat. No. 4,719,480 teaches methods and devices for spatially
stabilizing the crests of capillary waves on the free surface of a
liquid, such as ink, while U.S. Pat. No. 4,719,476 discloses
methods and devices emitting droplets from the crests of capillary
waves. In particular, U.S. Pat. No. 4,719,476 discusses ejecting
droplets from the crests using acousticaily induced secondary
capillary waves, heaters, laser beams, and ions. These techniques
for ejecting droplets from capillary waves may not be optimum.
What is needed are easily implemented methods and devices for
inducing droplets to be ejected from the crests of capillary waves.
Such methods and devices would be particularly useful in
drop-on-demand printers.
BRIEF SUMMARY OF THE INVENTION
The present invention provides for parametrically pumping droplets
from the crests of capillary waves using electric fields. Capillary
waves are generated on the free surface of a liquid, beneficially
by using acoustical energy, at a level approaching the onset of
droplet ejection (see Eisenmenger, "Dynamic Properties of the
Surface Tension of Water and Aqueous Solutions of Surface Active
Agents with Standing Capillary Waves in the Frequency Range from 10
kc/c to 1.5 Mc/s," ACUSTICA, volume 9, 1959, pages 327-340,
specifically page 335). To eject a droplet, an electric field
parametrically pumps the liquid such that sufficient energy is
imparted to the capillary wave that ejection occurs. To assist
pumping, the spatial positions of the crests are preferably
stabilized with respect to the container holding the liquid.
According to one embodiment, the electric field that pumps the
liquid is time varying and synchronized with the motion of the
crest. To reduce the effects of the electric field on the ejected
droplet, the electric field beneficially drops to zero at about the
time that the droplet is ejected. One application of the present
invention is in an inventive print head useful for drop-on-demand
printing.
BRIEF DESCRIPTION OF THE DRAWINGS
Other aspects of the present invention will become apparent as the
following description is read in conjunction with the following
drawings, in which:
FIG. 1 shows a simplified, fragmentary, isometric view of a
plurality of droplet ejectors according to one embodiment of the
present invention;
FIG. 2 shows stabilizing grooves for spatially stabilizing the
capillary wave crests that are formed into the inner sidewall of
the embodiment of FIG. 1;
FIG. 3 illustrates the spatial relationship between the capillary
wave crests and the probes according to the embodiment of FIG. 1;
and
FIG. 4 illustrates the relationship between the wave crest motion,
the electric field, and the attractive force produced by the
electric field according to the embodiment of FIG. 1.
DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT
While the present invention is hereinafter described in connection
with illustrated embodiment droplet ejectors and in connection with
a print head useful for drop-on-demand printing, it is to be
understood that the present invention is not limited to that
embodiment or application. On the contrary, the present invention
includes all alternatives, modifications and equivalents, either
now known or as may become known, as may be included within the
spirit and scope of the appended claims.
The present invention provides for ejecting droplets from the
crests of capillary waves riding on the free surface of a suitable
liquid, beneficially a marking fluid such as ink, by using electric
fields. According to one embodiment, the amplitudes of the
capillary wave crests approach the level at which droplets are
self-ejected from the crests, hereinafter referred to as the
droplet ejection threshold. The crests are beneficially spatially
fixed within the container holding the marking fluid. When a
droplet is to be ejected, the marking fluid within the capillary
wave is attracted by an electric field such that the attractive
force parametrically pumps the fluid to a level whereby a droplet
is ejected. The term "parametric pumping" as used herein refers to
the adding of energy to the system by forces synchronized with the
capillary wave motion. Preferably, the pumping electric field
approaches a zero magnitude about the time the droplet is
ejected.
THE DROPLET EJECTOR ASSEMBLY
One apparatus for practicing the present invention is shown in FIG.
1, a simplified, fragmentary isometric view of a printer which
includes a plurality of individual droplet ejectors. It is to be
understood that in the illustrated embodiment that the droplet
ejectors are separated by a distance which corresponds to the
desired pixel (picture element) resolution of the printer. The
droplet ejectors share a common container 4 formed by a first wall
6, a second wall 18, an electrically conductive bottom plate 10,
and two ends (not shown). The container 4 has an inner channel 12
formed by the first and second walls and holds a marking fluid 14,
such as a water based ink, in the channel. In the embodiment
illustrated in FIG. 1, the first and second walls are both slabs of
single crystal silicon, each about 81/2 inches long by 0.1 inch
thick by 0.25 inch high. The top surface 16 of each wall slopes
downwardly at about a 15 degree angle to form beveled lips 18 (see
FIG. 2), useful in retaining the marking fluid 14 within the
channel 12. The walls are arranged with their beveled lips facing
each other across the width of the channel 12, typically separated
by about 0.1 millimeter.
Still referring to FIG. 1, a transducer 20 generates ultrasonic
energy which creates capillary waves on the free surface 22 of the
marking fluid 14, as discussed in U.S. Pat. No. 4,719,476. The
output of the transducer is beneficially adjusted so that the
resulting wave amplitude is slightly below the droplet ejection
threshold. Referring now to FIGS. 1 and 2, the resulting capillary
wave has a plurality of wave crests 24 which are spatially
stabilized within the container 4 by vertical grooves 26 formed
into the inner side 28 of the first wall 6. By spatially stabilized
it is meant that the crests occur at fixed locations, even though
the wave surface varies from a crest to a trough at those fixed
locations. In one embodiment these grooves are formed by
anisotropic etching. Similar stabilizing grooves are discussed in
U.S. Pat. No. 4,719,480.
Referring now to FIGS. 1 and 3, the individual droplet ejectors
also include a pair of electrically conductive probes 30 located
such that each probe is slightly above, but on opposite sides, of
an associated stabilized crest 24. The probe pairs electrically
connect, via wires 32, to a controller 34 which selectively applies
voltage from a source 36 to the probe pairs. The source return is
through the marking fluid 14 via an electrical connection 37 made
with the conductive bottom plate 10. In the embodiment of FIG. 1,
the individual probes of a probe pair are mounted within apertures
38 of a fixed, insulating glass plate 40 which is disposed slightly
above the container 4. The glass plate helps locate the probes
adjacent the crests, white the apertures permit ejected droplets to
leave the vicinity of the probes. In response to signals from an
image source 42, the controller selectively connects the source 36
to the probe pairs of individual droplet ejectors as required to
eject droplets to produce an image on a recording medium 44 as the
recording medium passes above the glass plate 40.
THE EJECTION PROCESS
The present invention is meant to be used with a liquid which
supports capillary waves. Capillary waves are characterized by
having restoring forces dominated by the surface tension of the
liquid on which they exist. The liquid used with the present
invention must also must be attracted by an electric field. That
liquids are attracted to electric field is well known, see, for
example, Martin Plonus' work "APPLIED ELECTROMAGNETICS," Chapter 5,
1978 edition. Tests indicate that common tap water in the City of
Palo Alto, Calif., having a viscosity of about 0.9 cp (centipoise),
a surface tension of about 72 dyne-cm (dynes per centimeter), and
an unknown but appreciable conductivity is usable with the
inventive method. Additionally, it is believed that a dye-based ink
having a surface tension of about 1.6 cp, a viscosity of about 55
dyne-cm, and an unknown but existent conductivity (properties
similar to ink commercially available for use with the
Hewlett-Packard Deskjet printer) is also usable. Finally, it is
believed that the number of liquids usable with the present
invention is very large.
To eject a droplet from the embodiment shown in FIG. 1, which
includes a capillary wave at an amplitude approaching the droplet
ejection threshold, only a relatively small amount of energy need
be added to the system. One way to add energy would be to apply a
sufficiently high voltage pulse to the probes 30, relative to the
conductive plate 10. That voltage would produce an attraction force
between the marking fluid 14 in the crest and the probe, thereby
inducing some of the fluid to be ejected and pass through the
aperture 38.
A way of ejecting droplets is to apply an electric field to the
probes 30 which parametrically pumps the marking fluid. One
technique for accomplishing this is described with the assistance
of the timing diagram of FIG. 4. In FIG. 4, the surface velocity of
the capillary wave is illustrated by trace 100 while the surface
height of the marking fluid is illustrated by trace 102. As
indicated, the capillary wave cyclically rises and falls at
positions fixed by the stabilizing grooves, resulting in crests and
valleys which occur at intervals dependent upon the capillary wave
frequency. To parametrically pump the marking fluid, the source 36
applies an alternating voltage, illustrated by trace 104, to the
probes 30, producing electric fields from the probes which pass
into the marking fluid 14. The electric field creates an attractive
force, illustrated by trace 106, that at a fixed distance from the
probes is proportional to the square of the electric field. With
reference to FIG. 4, at time A the attractive force between the
probes 30 and the marking fluid is zero and the capillary wave
surface height is at its maximum. Between times A and B, the
attractive force (at a fixed distance from the probes) increases as
the surface height falls until the attractive force reaches a
maximum when the surface height is at its minimum. Between times A
and C, the attractive force draws the marking fluid toward the
probes, imparting energy to the system. As shown, the attractive
force decreases as the surface height approaches the probes. At
time C, about when a droplet is ejected, the attractive force is
zero. Provided that the attractive force has imparted sufficient
energy to the system, the surface tension of the marking fluid at
the crest is overcome and a droplet is ejected. If another droplet
is to be ejected, the process repeats as shown by times C, D, and
E.
With the probes 30 symmetrically disposed about the crests 24 as
illustrated in FIG. 3, the effects of their attractive forces on
the crest are substantially additive. This is beneficial because
the attractive forces then cause the ejected droplet to pass more
or less centrally between the probes and through the aperture 38,
provided that sufficient energy has been imparted to the system and
that the attractive forces on the droplet are small when the
droplet passes the probes.
From the above, it is clear that many factors should be considered
when implementing the above described embodiment. The
characteristics of the marking fluid, the probe position, the
voltage applied to the probes, the intensity and frequency of the
ultrasonic energy from the transducer, the synchronization between
the voltage applied to the probes and the wave crests, and the
depth of the marking fluid all should be balanced.
While the foregoing described an improved method and apparatus for
selectively ejecting droplets from capillary waves and the use of
the method and apparatus in a printer, it is evident that many
alternatives, modifications and variations will be apparent to
those skilled in the art. Accordingly, it is intended that the
present invention embrace all alternatives, modifications and
variations that fall within the spirit and scope of the appended
claims.
* * * * *